Apple continuously enhanced the security of recent macOS versions and with High Sierra 10.13 it introduced a new feature that requires user approval before loading newly-installed third-party kernel extensions (KEXTs)

There are lots of complaints from users not being able to install applications with KEXT, like Oracle VirtualBox. The mechanism is relatively complex, but basically each developer is required to get a developer ID from Apple, and users can approve that software signed by that developer is allowed to run.Read more of this post